While investigating parameters of the regulatory function of CO2 on procaryot and eucaryot cell growth, we uncovered an unusual physiological effect on cell division caused by deletion of CO2. When CO2 is eliminated from actively growing cultures of Escherichia coli, gross protein synthesis is inhibited, but cell division is not inhibited; it continues for one population doubling. The resulting cells are approximately half normal size. The current model of the cell cycle is based upon experimental evidence illustrating the independent completion of replication of incompleted rounds of chromosome synthesis followed by a period of protein synthesis to permit cells to acquire their pre-cell division size and complement of protein. This is followed by a period of additional protein synthesis during which the signal protein for cell division is produced. Our system offers the possibility of isolated direct study of the cell division signal divorced from a requirement for preliminary increase in cell size. BIBLIOGRAPHIC REFERENCES: Repaske, R. and Repaske, A.C: Quantitative requirements for exponential growth of Alcaligenes eutrophus. Appl. & Environ. Bacteriol. 32: 585-591, 1976. Repaske, R. and Mayer, R.: Dense autrophic cultures of Alcaligenes eutrophus. Appl. & Environ. Bacteriol. 32: 592-597, 1976.